Understanding the data flow from memory to microprocessor and the instruction executed by the microprocessor

The instructions which are to be executed by the microprocessor are first stored in the memory of the processor and then executed. But the processor does not execute the instructions directly. It reads the instruction byte by byte and then executes it.
Before understanding instruction execution and data flow, there is a necessity for us to learn a new term, OPCODE.

What is an Opcode?
Opcode is nothing but the machine language instruction which denotes the microprocessor about what operation should be performed on the specific data. Microprocessor converts the instruction into suitable machine language, so that it can understand the operation to be performed and executes it.

Consider MVI A, 18H.
When the above instruction is to be executed, the microprocessor gets the Opcode for MVI A and performs the necessary operation on the data which is 18H in this case. The Opcode for MVI A is 3EH. So the microprocessor first reads this Opcode from the instruction and then performs the operation specified by Opcode over the data given.

Now let us assume we want to store the above instruction in a specific address say 5500H. We know that in 8085 processor only one byte can be stored in each address location. Therefore the Opcode 3EH is stored at the location 5500H and the data 18H is stored at the next location 5501H.

Now for execution of this instruction the processor has to send the address to the memory for reading. Then the MEMR’ signal is activated. As soon as this signal is activated the memory places the Opcode byte (3EH) on the data bus.

The above process is considered as a single cycle and is called the OPCODE FETCH CYCLE. The period during which the Opcode is fetched from address to the data bus is called as Opcode fetch cycle.


A microprocessor by default knows that the first byte which is under execution is always Opcode. The internal data bus sends the Opcode to the instruction decoder. The instruction decoder decodes the Opcode and identifies it as MVI A instruction. As soon as this information is obtained, the microprocessor searches for the data on which this operation should be performed.

To find the data for performing the operation, the microprocessor instructs the timing and control unit to generate a proper timing signal to obtain the data. As a result of the timing signal, the program counter is increased by 1. So the address bus moves from 5500H to 5501H. Now we know that the data 18H is placed at 5501H. So when the address bus is placed at 5501H, it identifies the data and the MEMR’ signal is activated. After the activation of this signal the data is placed on the internal address bus and then it is moved to the accumulator. Then the MVI A operation is performed on the data 18H and the result is sent to the respective registers.

This process of placing the address and reading the data is considered as a single cycle and this cycle is called Memory Read cycle. In general these cycles are called as machine cycles.

Thus we have successfully discussed the data flow from memory to microprocessor and how the instruction is executed by the microprocessor.

Comments

Post a Comment

Popular posts from this blog

FPGA In Layman''S Terms

Image
FPGA is an Integrated Circuit which can be reconfigured again and again to perform different tasks as desired. For example, an FPGA can be configured to function as a processor, and then it can be reconfigured to function as a Graphics Processing Unit (GPU), or hardware video encoder, or something else. The point is that FPGA can theoretically be programmed to function as any other Digital Integrated Circuit. Think of FPGA as a breadboard for Digital Circuits. All the gates and flip-flops are present on the FPGA, and there are wires going through the complete chip. Circuits are made by connecting these wires to the relevant gates or flip-flops as per intended design. Instead of connecting the gates/flip-flops using a physical wire as in a breadboard, FPGAs has programmable interconnects that can be rewired programmatically. There are dedicated wires/routes for clock signals and only a selected number of FPGA pins are allowed to drive those global clock routing wires. FPGA Vs Microcontr...
Read more

BOARD BRING-UP

Board bring-up is the process of checking and validating a PCB assembly for the first time. The aim is to make sure that the board has been manufactured and assembled correctly and that all of the subsections of the design work as intended. This can be a nervous time for an electronics designer as it is a key proof point that many weeks or months of work has been successful. Note that bring-up is not the same as validating an entire product or system – it is usually limited to validating the hardware. Writing firmware will likely be a part of the process, but only to the extent required to ensure the processor, its peripherals and external circuitry connected to them are working as expected. Board Bring-up process. 1. Visual Inspection: Print out assembly drawing with all the components and test points to have a quick reference during validation. All the components are placed correctly. All the PINs 1 are placed correctly. There are no visible short circuits between PADS. Are pads sold...
Read more

High Speed Digital Design Considerations

Image
The four main consideration of today’s high-speed design:                                                                                                                                                            Ref(1) We need to consider signal integrity , which includes parameters such as Overshoot and Ringing. Need to look at crosstalk which is about EMI coupling between traces on PCB board Timing is about calculating the flight time EMI and radiation  These parameters are related to each other and aff...
Read more